There is an option in the weapon bar config menu to turn on auto-hiding of the bar. The bar will stay hidden most of the time, until your weapon buff expires, in which case it will show again until you recast one.

There is a similar feature for the shield bar, however since blizzard doesn't allow hiding or showing of bars during combat, instead of the bar physically disappearing, the relevant shield icon will change to indicate which shield is active, and go back to normal when the shield buff wears off.

To install, copy the ShamanBuffBars folder into your Addons directory.

For example: C:/Program Files/World of Warcraft/Interface/Addons/ShamanBuffBars